Simultaneity - Albert Einstein and the Theory of Relativity

Imagine two observers, one seated in the center of a speeding train car, and another standing on the platform as the train races by. As the center of the car passes the observer on the platform, he sees two bolts of lightning strike the car - one on the front, and one on the rear. The flashes of light from each strike reach him at the same time, so he concludes that the bolts were simultaneous, since he knows that the light from both strikes traveled the same distance at the same speed, the speed of light. He also predicts that his friend on the train will notice the front strike before the rear strike, because from her perspective on the platform the train is moving to meet the flash from the front, and moving away from the flash from the rear.

But what does the passenger see? As her friend on the platform predicted, the passenger does notice the flash from the front before the flash from the rear. But her conclusion is very different. As Einstein showed, the speed of the flashes as measured in the reference frame of the train must also be the speed of light. So, because each light pulse travels the same distance from each end of the train to the passenger, and because both pulses must move at the same speed, he can only conclude one thing: if he sees the front strike first, it actually happened first.

Whose interpretation is correct - the observer on the platform, who claims that the strikes happened simultaneously, or the observer on the train, who claims that the front strike happened before the rear strike? Einstein tells us that both are correct, within their own frame of reference. This is a fundamental result of special relativity: From different reference frames, there can never be agreement on the simultaneity of events.

• Category

  News & Politics

[libalchris]

No and No.

Simultaneity doesn't deal with single events (nothing in relativity suggests individual space-time events are frame-dependent, that would be absurd), it deals with two or more separate events, and what space-time coordinates they occur at. While both spatial and temporal separation are frame dependent, the space-time interval ∆s^2=-c∆t^2+∆r^2 is not.

Simultaneity means a set of events occur at the same time. Time itself is a dimension, though one different from spatial dimensions.

[mraccident]

The question is - do you think that there is a single event that looks different from different perspectives; or that the event actually changes, depending on the observer?

If it's the latter - it seems like a belief to me.

Also - do you think that "time" and "simultaneity" are the same thing?

[libalchris]

Yes, but synchronize them to each other in what frame of reference? The synchronicity of the clocks is frame-dependent.

Two events can be specified by their space-time coordinates. Though there is an invariant quantity relating coordinates between different frames, the actual values of the coordinates depend on the frame of reference. Only in one frame can two events at different coordinates have the same time coordinate, thus being "simultaneous." Therefore, simultaneity is relative.

[mraccident]

They don't need to be synchronize in both frames; only to each other.

[libalchris]

The constant speed of light is exactly why this has to happen, actually. Ground observer sees the train observer moving away from the back bolt and toward the front one. This means in order for the light from each bolt to reach her at the same time, the light from the back bolt would have to be travelling faster than the light from the front one.

For a constant speed of light, the light from each bolt must reach her at different times, and so she must conclude they stuck at different times.

[libalchris]

it is imposible to synchronize the clocks in both frames. If they are synchronous in one frame, they will be unsynchronized in the other.

With relativity, "who's doing the moving" is irrelevant, as all motion is relative. What matters is who's frame the clocks are synchronized with, since they cannot be synchronized with both. The point of all this is that two events that one observer sees as simultaneous, may not be simultaneous for another observer

[shield543]

Suppose one observer concludes that The Big Bang and "Event X" occurred simultaneously, while another observer may conclude that "Event X" occurred after/before the Big Bang... doesn't this bring up multiple questions?? Or is it just because this falls under Special Relativity and I am missing something from General Relativity? (Which I am unfamiliar with)

[tyler marchant]

this video dosnt make any sense albert Einstein proved that anything moving under the speed of light will get passed by a light beam at the full speed of light for example a race car at 200 mph and a person standing still will get passed at the full 186000 miles per second the race cars speed means absolutely nothing so the speed of the train means nothing on a light speed perspective so they would both see the lightning bolts hit the train simultaneously and shafiq your a nobody quit trolling

[Giovanni Bonin]

You are the most ignorant and arrogant person I have ever seen on Youtube. Beware, that's a really competitive title, so congratulations.